An integrated circuit (IC) to supply power to an active matrix organic light emitting diode (AMOLED) (hereinafter, referred to as a power supply apparatus) stably supplies power to the AMOLED. In most cases, such an IC includes a buck or boost converter for forming a positive voltage in accordance with the condition of an input voltage, and an inverter for forming a negative voltage in accordance with the condition of the input voltage. The IC uses internal elements such as an NMOS transistor at a positive stage and a PMOS transistor at a negative stage.
In practical application of the IC, the positive and negative stages form opposite voltages for an OLED, respectively. The amount of current flowing through the output-stage transistor of the power supply apparatus directly determines the magnitude of power. Different from a linear device, the power supply apparatus operates in a switch mode, in order to achieve an enhancement in efficiency. Ideally, the power supply apparatus should exhibit a constant efficiency irrespective of the magnitude of a load. In practical cases, however, internal power loss is inevitably generated due to the circuit and system used in the power supply apparatus.
The power loss of the power supply apparatus may be mainly divided into switching loss and conduction loss. Where a power transistor having a large capacity is used as a switch, the conduction loss is proportional to the size of the power transistor. In order to reduce the conduction loss, accordingly, the power supply apparatus uses a high switching frequency. In this case, however, an increase in switching loss occurs. Since there is a trade-off between switching loss and conduction loss, an appropriate switching frequency should be used, and the power transistor should have an appropriate size.
The power loss in a general power supply apparatus may be expressed by the following Expression 1:Pavg=Pswitching+Pshort-circuit+Pleakage+Pstatic 
In Expression 1, “Pswitching” represents a switching component of the power, namely, a power loss caused by a charge/discharge parasitic capacitance, and corresponds to about 70 to 90% of the overall loss. “Pshort-circuit” represents a power loss caused by an ON resistance of the power transistor, and corresponds to about 10 to 30% of the overall loss. “Pleakage” represents a leakage component of the power, and corresponds to about 1% or less of the overall loss. “Pstatic” represents static power.
In a circuit using an ideal MOS transistor, there is little or no static power. Referring to Expression 1, it can be seen that the power loss caused by the switching power “Pswitching” is largest in the average power “Paverage” of the power supply apparatus.
Nevertheless, in most cases, the size of the power transistor is preferentially taken into consideration, upon designing the power supply apparatus, in order to drive maximal power. For this reason, the power supply apparatus uses an increased frequency. Although optimal design is possible under a normal load condition, there may be a problem in that, under a low-load condition, degradation in efficiency inevitably occurs when the frequency is high.